Micromirror array devices have been developed because it can substitute for conventional optoelectronic devices. Phase-only piston-style micromirrors were used for phase adaptive optics applications, and rotational micromirrors were used to deflect light. These micromirrors have been controlled by analog control to have continuous displacement, which is determined at the equilibrium between electrostatic force and elastic force. The analog control is more complex than the digital or discrete control, and is not compatible with known semiconductor electronics technologies such as MOS, CMOS, etc.
In the prior art of a micromirror array such as the Digital Micromirror Device in U.S. Pat. Nos. 4,566,939, 5,083,857, and 6,232,936, each micromirror is actuated by digital voltage control and can have large rotation with low driving voltage. It is compatible with known semiconductor electronics technologies but has only single degree of freedom; rotation about an axis with two level positions. So, the micromirror array is merely the array of optical switches.
The capability of the micromirror array is not limited to only optical switches. The micromirror array can be used to simulate curved optical surfaces by changing its surface profile. The micromirror array described in J. Boyd and G. Cho, 2003, “Fast-response Variable Focusing Micromirror Array Lens,” Proceeding of SPIE Vol. 5055: pp. 278-286 can provide axis-symmetric lenses, wherein every mirror at given radius is controlled by one pair of a circular electrode and a circular ground electrode. However, this micromirror array has difficulties to make a micromirror array have a desired surface profile simulating such as aspherical (e.g. parabolic, hyperbolic, elliptical, etc), or non-axis-symmetric lenses.
Some micromirrors are provided with more degrees of freedom using multiple electrodes. U.S. Pat. No. 6,687,112 to Chiou discloses a control system having a rotating and a translating actions for an electrostatically-driven micro-electromechanical device comprising a movable plate, multiple electrostatically-driving electrodes, and a controller to determinate operation characteristics of the electrostatically-driven micro-electromechanical device and selecting electrode patterns through a switching matrix circuit. In this system, the attractive forces are applied to micromirror itself, which may cause a structural deformation due to residual stress leading to the degradation of the image quality. Also, the variation of the height of the electrodes in the direction of translational motion is not utilized while the variation of the height of the electrodes can provide the effective control of the micromirror. Further, this system does not provide a micromirror array system capable of providing multiple surface profiles.
U.S. Pat. No. 6,906,848 to Aubuchon discloses micromirror systems providing a micromirror array wherein at least one of electrodes is configured with a plurality of portions at different levels, so that portions further from a center of rotation of a micromirror are at a greater distance from the micromirror than portions closer to the center of rotation for sequential attraction of the micromirror portion. This system, however, is operated in the analog mode, which requires complex and fine control of voltage and can cause a snap down problem. Considering the number of micromirrors in the micromirror array which can be reached to tens of millions and the number of electrodes required for each micromirror, digitally or discretely controlled micromirror array systems can be used more advantageously. Further, the attractive forces are applied to micromirror itself, which may cause a structural deformation due to residual stress leading to the degradation of the image quality.
To be applied more widely, a discretely controlled micromirror array device is required to provide multiple surface profiles, low structural deformation, and compatibility with known semiconductor electronics technologies.